Fractures related to osteoporosis continue to grow at an alarming rate, despite advances in bone biology and pharmaceutical treatment. The situation suggests the need for bold, new strategies for both the treatment and the prevention of osteoporosis. To be successful, such strategies must be based in a sound understanding of the relevant biological mechanisms and sufficiently simple and low cost to be applicable on a national scale to a broad segment of the population. Given that: 1) peak bone mass is an important determinant of fracture risk in later life; 2) up to 40 percent of peak bone mass can be influenced by environmental factors such as exercise and diet; and 3) childhood and prepubescence are the most critical time periods for skeletal mineralization, we conducted during the previous funding period what we believe to be the first randomized controlled study investigating the effects of a highly specific jumping program on bone mass in pre-pubertal children. We showed that 7 months of high-impact loading from jumping off two foot boxes was both safe and effective and resulted in 4.5 percent and 3.1 percent (p less than 0.01) increases compared to controls in bone mineral content (BMC) at the hip and spine, respectively. Moreover, after an additional 7 months of detraining, jumpers maintained a 4 percent greater femoral neck BMC (p less than 0.05). After detraining, the group differences at the spine, although positive, were not statistically significant. The exercise program was simple, practical and low cost enough so that it could be incorporated into elementary school physical education curricula. In this competitive renewal, we now propose to build on and extend these findings by determining: 1) if two sequential, 7-month high- impact jumping interventions can be used to increase bone mass even further; 2) if the increases in bone mass are again maintained after detraining; and 3) if the increases in bone mass observed in the subjects studied during the previous funding period continue to be maintained through puberty and into adolescence. As we have done previously, we will measure ground impact forces from jumping in individual subjects both early and late in the intervention, with the advantage now that dual force plates can be used to determine forces in each leg. Given the relatively stable nature of the Corvallis population, we believe that our program represents a valuable and unique resource for the long-term study of exercise effects in the developing skeleton. We also believe our program can serve as a model for exercise-based interventions that might be implemented successfully on regional and even national levels. Thus, if we continue to produce and maintain practically important and structurally significant increases in bone mass at the hip and spine in growing children, we believe we will have taken an important step toward the bold new strategies that are needed if we are to reverse the continued increases in the numbers of age- related fractures.